The present invention relates to signal equalization in a modem for use in a digital communication system, and, more particularly, to an algorithm control method for a blind equalizer.
In a digital communication system, a transmitter inserts a predetermined training sequence into a transmission signal at a certain interval, and transmits the inserted signal to a receiver. The receiver, on the other hand, detects and recognizes the training sequence, thereby discriminating a pattern of the transmitted signal to perform signal equalization. However, the transmitter may not transmit the transmission signal together with the training sequence. In this case, the receiver cannot recognize the pattern and state of the received signal. A method for receiving and equalizing the transmission signal which does not include the training sequence is called a blind equalization. An equalizer of such a blind equalization system is used in a voice band modem or a modem for digital communications such as a full digital high definition television, etc.
Algorithms used in the blind equalization process include a Godard algorithm, a stop-and-go algorithm (SGA), and a decision-directed algorithm (DDA), as examples. The Godard algorithm is discussed in the reference, "Self-recovering Equalizer and carrier Tracking in Two Dimensional Data Communication System" by D. N. Godard (IEEE Transactions on Communication, Volume COM-28, No. 11, pp. 1867-1875, November 1980).
FIG. 1 is a block diagram showing a general blind equalization system. Generally, since the DDA is not converged until the channel distortion of the received signal is removed to a degree, the initial employment of the DDA can cause the equalization to fail. Thus, the Godard algorithm which can obtain excellent convergency even if the channel distortion is severe should be firstly executed and then a fine equalization should be executed with the DDA. In the blind equalization system, a Godard algorithm execution circuit 13 performs equalization using the Godard algorithm with respect to the applied unequalized signal Y.sub.n. When the number of signal equalizations using the Godard algorithm reaches a predetermined number, the blind equalization system controls a DDA execution circuit 14 to perform the signal equalization. Since then, the applied signal Y.sub.n is minutely equalized by the DDA of the DDA execution circuit 14, the SGA, or a modified SGA. In this manner, a coefficient C.sub.n which is obtained by the Godard algorithm execution circuit 13 or DDA execution circuit 14 is used for updating tap-weights of the blind equalizer 11.
However, in the conventional blind equalization system, the Godard algorithm is used a predetermined number of times and is converted into the DDA. Accordingly, the equalization becomes worse. Also, since a step size of the equalization coefficient is uniquely applied in each algorithm, stable convergency of the equalizer cannot be easily obtained.
European Patent Laid-open Publication No. 0,524,559 A2 by Paik et al., published on Jan. 27, 1993, attempts to solve these problems. This reference discloses the use of a constant modulus algorithm (CMA) which is similar to the Godard algorithm to initialize the equalization coefficients. The DDA is performed when the phase error of the signal equalized by the equalization coefficients matches a predetermined threshold value. If the phase error does not match the threshold value while performing the DDA, the CMA is again performed.